Materials:
Milk or Juice cartons
Knife and Scissors
Potting Soil

optional
Brush
Acrylic paint
or
Fabric
Glue
A foam brush (or a sponge)

Instructions
Clean the container(s)

Decide how deep you want the container to be, and draw a line around the milk carton where you want to cut. Use sharp scissors or a serrated knife to cut out the top of the milk carton.

Optional: cut a cute design, like ears or other things on the top

Optional: Paint the carton or wrap it in fabric

a. You may need to coat it more than once, allow the paint to fully dry between coats and before continuing.

b. Using a sponge spread white glue over the carton and wrap the carton with the fabric, cut the corners of the fabric to fold easily around the bottom. Put a little glue on the top edge to avoid fabric to fray.

Fill the planter with potting soil, up to ¼ inch from the top. Plant your seed according to the packet and cover the seed with some soil. Mist the soil and place the carton on a sunny windowsill.

For more room you can lay the carton on its side and cut the topside off

To avoid overwatering you have two options.

First, you can punch a hole in the planter bottom so that water drains.
Use a screwdriver, knife, or scissors to poke a couple of holes in the bottom of the carton. This will allow any excess water to drain out of your planter.

The second option is that you can add some pebbles in the planter bottom before adding the soil. It will prevent the roots from soaking in water and rotting.

You could also add flowers instead of seeds.